Multi-parameter arrhythmia discrimination

ABSTRACT

An arrhythmia discrimination device and method involves receiving electrocardiogram signals and non-electrophysiologic signals at subcutaneous locations. Both the electrocardiogram-signals and non-electrophysiologic signals are used to discriminate between normal sinus rhythm and an arrhythmia. An arrhythmia may be detected using electrocardiogram signals, and verified using the non-electrophysiologic signals. A detection window may be initiated in response to receiving the electrocardiogram signal, and used to determine whether the non-electrophysiologic signal is received at a time falling within the detection window. Heart rates may be computed based on both the electrocardiogram signals and non-electrophysiologic signals. The rates may be used to discriminate between normal sinus rhythm and arrhythmia, and used to determining absence of an arrhythmia.

RELATED APPLICATIONS

[0001] This application claims the benefit of Provisional PatentApplication Serial No. 60/462,272, filed on Apr. 11, 2003, to whichpriority is claimed pursuant to 35 U.S.C. §119(e) and which is herebyincorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to implantable cardiacmonitoring and stimulation devices and, more particularly, tomulti-parameter arrhythmia discrimination using electrocardiograminformation and non-electrophysiological cardiac activity information.

BACKGROUND OF THE INVENTION

[0003] The healthy heart produces regular, synchronized contractions.Rhythmic contractions of the heart are normally controlled by thesinoatrial (SA) node, which is a group of specialized cells located inthe upper right atrium. The SA node is the normal pacemaker of theheart, typically initiating 60-100 heartbeats per minute. When the SAnode is pacing the heart normally, the heart is said to be in normalsinus rhythm.

[0004] If the heart's electrical activity becomes uncoordinated orirregular, the heart is denoted to be arrhythmic. Cardiac arrhythmiaimpairs cardiac efficiency and can be a potential life-threateningevent. Cardiac arrhythmias have a number of etiological sources,including tissue damage due to myocardial infarction, infection, ordegradation of the heart's ability to generate or synchronize theelectrical impulses that coordinate contractions.

[0005] Bradycardia occurs when the heart rhythm is too slow. Thiscondition may be caused, for example, by impaired function of the SAnode, denoted sick sinus syndrome, or by delayed propagation or blockageof the electrical impulse between the atria and ventricles. Bradycardiaproduces a heart rate that is too slow to maintain adequate circulation.

[0006] When the heart rate is too rapid, the condition is denotedtachycardia. Tachycardia may have its origin in either the atria or theventricles. Tachycardias occurring in the atria of the heart, forexample, include atrial fibrillation and atrial flutter. Both conditionsare characterized by rapid contractions of the atria. Besides beinghemodynamically inefficient, the rapid contractions of the atria mayalso adversely affect the ventricular rate.

[0007] Ventricular tachycardia occurs, for example, when electricalactivity arises in the ventricular myocardium at a rate more rapid thanthe normal sinus rhythm. Ventricular tachycardia can quickly degenerateinto ventricular fibrillation. Ventricular fibrillation is a conditiondenoted by extremely rapid, uncoordinated electrical activity within theventricular tissue. The rapid and erratic excitation of the ventriculartissue prevents synchronized contractions and impairs the heart'sability to effectively pump blood to the body, which is a fatalcondition unless the heart is returned to sinus rhythm within a fewminutes.

[0008] Implantable cardiac rhythm management systems have been used asan effective treatment for patients with serious arrhythmias. Thesesystems typically include one or more leads and circuitry to sensesignals from one or more interior and/or exterior surfaces of the heart.Such systems also include circuitry for generating electrical pulsesthat are applied to cardiac tissue at one or more interior and/orexterior surfaces of the heart. For example, leads extending into thepatient's heart are connected to electrodes that contact the myocardiumfor sensing the heart's electrical signals and for delivering pulses tothe heart in accordance with various therapies for treating arrhythmias.

[0009] Typical Implantable cardioverter/defibrillators (ICDs) includeone or more endocardial leads to which at least one defibrillationelectrode is connected. Such ICDs are capable of delivering high-energyshocks to the heart, interrupting the ventricular tachyarrhythmia orventricular fibrillation, and allowing the heart to resume normal sinusrhythm. ICDs may also include pacing functionality.

[0010] Although ICDs are very effective at preventing Sudden CardiacDeath (SCD), most people at risk of SCD are not provided withimplantable defibrillators. Primary reasons for this unfortunate realityinclude the limited number of physicians qualified to performtransvenous lead/electrode implantation, a limited number of surgicalfacilities adequately equipped to accommodate such cardiac procedures,and a limited number of the at-risk patient population that may safelyundergo the required endocardial or epicardial lead/electrode implantprocedure.

SUMMARY OF THE INVENTION

[0011] The present invention is directed to cardiac monitoring and/orstimulation methods and systems that, in general, provide transthoracicmonitoring, defibrillation therapies, pacing therapies, or a combinationof these capabilities. Embodiments of the present invention are directedto subcutaneous cardiac monitoring and/or stimulation methods andsystems that detect and/or treat cardiac activity or arrhythmias.

[0012] An embodiment of the invention is directed to an arrhythmiadiscrimination method that involves sensing electrocardiogram signals ata subcutaneous non-intrathoracic location. The electrocardiogram signalsmay include a cardiac signal and one or both of noise andelectrocardiographic artifacts. Signals associated with anon-electrophysiological cardiac source are also received. A sensedelectrocardiogram signal is verified to be a cardiac signal using anon-electrophysiological signal. A cardiac arrhythmia is detected usingone or both of the sensed electrocardiogram signal and the verifiedcardiac signal. Treatment of the cardiac arrhythmia is withheld if thesensed signal is not verified to be the cardiac signal.

[0013] An arrhythmia may be detected using the electrocardiogramsignals, and the presence of the arrhythmia may be verified or refutedusing the non-electrophysiologic signals. Temporal relationships betweenthe electrocardiogram signals and non-electrophysiologic signals may bedetermined. A detection window may be initiated in response to receivingthe electrocardiogram signal, and used to determine whether thenon-electrophysiologic signal is received at a time falling within thedetection window.

[0014] Heart rates may be computed based on both a succession ofelectrocardiogram signals and a succession of non-electrophysiologicsignals. The rates may be used to discriminate between normal sinusrhythm and the arrhythmia. The rates may be compared with arrhythmiathresholds, and used to determine absence of an arrhythmia, such as inresponse to a first rate exceeding a first arrhythmia threshold and asecond rate failing to exceed a second arrhythmia threshold. Thepresence of an arrhythmia may be determined using a morphology of theelectrocardiogram signals, and then verified using thenon-electrophysiologic signals. Examples of non-electrophysiologicsignals include heart sound signals, subsonic acoustic signalsindicative of cardiac activity, pulse pressure signals, impedancesignals indicative of cardiac activity, and pulse oximetry signals.

[0015] In another embodiment of the present invention, defibrillationtherapy delivery may be inhibited in response to detecting an arrhythmiausing the electrocardiogram signals but not detecting the arrhythmiausing the non-electrophysiologic signal. A method of sensing anarrhythmia and inhibiting therapy may involve sensing anelectrocardiogram signal at a subcutaneous non-intrathoracic location. Adetection window may be defined with a start time determined from theelectrocardiogram signal. A signal associated with anon-electrophysiological cardiac source may be received and evaluatedwithin the detection window. The presence or non-presence of a cardiacarrhythmia may be determined using the electrocardiogram signal, andconfirmed by the presence of the cardiac arrhythmia as detected by thenon-electrophysiological cardiac signal. The start time of a detectionwindow used for confirmation may be associated with an inflection pointof the electrocardiogram signal, such as a maxima or a minima. Acorrelation may be performed between the electrocardiogram signal andthe non-electrophysiological cardiac signal.

[0016] An embodiment of the present invention is directed to animplantable cardiac device including a housing and an electrodearrangement configured for subcutaneous non-intrathoracic placement.Detection circuitry is provided in the housing and coupled to theelectrode arrangement. The detection circuitry is configured to detectelectrocardiogram signals comprising a cardiac signal and one or both ofnoise and electrocardiographic artifacts. A sensor configured to sensenon-electrophysiologic signals associated with anon-electrophysiological cardiac source is coupled to the detectioncircuitry. A processor is provided in the housing and coupled to thedetection circuitry, sensor, and energy delivery circuitry, anddiscriminates between normal sinus rhythm and arrhythmia using theelectrocardiogram and non-electrophysiologic signals. The processorverifies that the sensed electrocardiogram signal is a cardiac signalusing the non-electrophysiological signal. The processor withholdstreatment of the cardiac arrhythmia if the sensed signal is not verifiedto include the cardiac signal.

[0017] The energy delivery circuitry may include one or both ofdefibrillation therapy circuitry and pacing therapy circuitry. Thesensor may be provided in or on the housing, and/or in or on a leadcoupled to the housing. Appropriate sensors include an accelerometer, amicrophone, an acoustic transducer, a blood-flow transducer,photoplethysmography circuitry, and a pulse oximeter.

[0018] The above summary of the present invention is not intended todescribe each embodiment or every implementation of the presentinvention. Advantages and attainments, together with a more completeunderstanding of the invention, will become apparent and appreciated byreferring to the following detailed description and claims taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIGS. 1A and 1B are views of a transthoracic cardiac sensingand/or stimulation device as implanted in a patient in accordance withan embodiment of the present invention;

[0020]FIG. 1C is a block diagram illustrating various components of atransthoracic cardiac sensing and/or stimulation device in accordancewith an embodiment of the present invention;

[0021]FIG. 1D is a block diagram illustrating various processing anddetection components of a transthoracic cardiac sensing and/orstimulation device in accordance with an embodiment of the presentinvention;

[0022]FIG. 2 is a diagram illustrating components of a transthoraciccardiac sensing and/or stimulation device including an electrode arrayin accordance with an embodiment of the present invention;

[0023]FIG. 3 is a pictorial diagram of a carotid pulse waveform, aphonocardiogram (PCG) waveform, an electrocardiogram (ECG) waveform, anda filtered transthoracic impedance signal for two consecutiveheartbeats;

[0024]FIG. 4 is a graph illustrating two consecutive PQRS complexes andtheir associated pseudo accelerometer signals, and a detection windowfor correlation of the signals in accordance with an embodiment of thepresent invention;

[0025]FIG. 5 is a flow chart illustrating methods of multi-parameterarrhythmia discrimination in accordance with the present invention; and

[0026]FIG. 6 is a flow chart illustrating a method of multi-parameterarrhythmia discrimination in accordance with the present invention.

[0027] While the invention is amenable to various modifications andalternative forms, specifics thereof have been shown by way of examplein the drawings and will be described in detail below. It is to beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0028] In the following description of the illustrated embodiments,references are made to the accompanying drawings, which form a parthereof, and in which is shown by way of illustration, variousembodiments in which the invention may be practiced. It is to beunderstood that other embodiments may be utilized, and structural andfunctional changes may be made without departing from the scope of thepresent invention.

[0029] An implanted device according to the present invention mayinclude one or more of the features, structures, methods, orcombinations thereof described hereinbelow. For example, a cardiacmonitor or a cardiac stimulator may be implemented to include one ormore of the advantageous features and/or processes described below. Itis intended that such a monitor, stimulator, or other implanted orpartially implanted device need not include all of the featuresdescribed herein, but may be implemented to include selected featuresthat provide for unique structures and/or functionality. Such a devicemay be implemented to provide a variety of therapeutic or diagnosticfunctions.

[0030] In general terms, a cardiac signal discrimination arrangement andmethod may be used with a subcutaneous cardiac monitoring and/orstimulation device. One such device is an implantable transthoraciccardiac sensing and/or stimulation (ITCS) device that may be implantedunder the skin in the chest region of a patient. The ITCS device may,for example, be implanted subcutaneously such that all or selectedelements of the device are positioned on the patient's front, back,side, or other body locations suitable for sensing cardiac activity anddelivering cardiac stimulation therapy. It is understood that elementsof the ITCS device may be located at several different body locations,such as in the chest, abdominal, or subclavian region with electrodeelements respectively positioned at different regions near, around, in,or on the heart.

[0031] The primary housing (e.g., the active or non-active can) of theITCS device, for example, may be configured for positioning outside ofthe rib cage at an intercostal or subcostal location, within theabdomen, or in the upper chest region (e.g., subclavian location, suchas above the third rib). In one implementation, one or more electrodesmay be located on the primary housing and/or at other locations about,but not in direct contact with the heart, great vessel or coronaryvasculature.

[0032] In another implementation, one or more leads incorporatingelectrodes may be located in direct contact with the heart, great vesselor coronary vasculature, such as via one or more leads implanted by useof conventional transvenous delivery approaches. In a furtherimplementation, for example, one or more subcutaneous electrodesubsystems or electrode arrays may be used to sense cardiac activity anddeliver cardiac stimulation energy in an ITCS device configurationemploying an active can or a configuration employing a non-active can.Electrodes may be situated at anterior and/or posterior locationsrelative to the heart. Examples of useful subcutaneous electrodes,electrode arrays, and orientations of same are described in commonlyowned U.S. patent application Ser. No. 10/738,608 entitled “NoiseCanceling Cardiac Electrodes,” filed Dec. 17, 2003, and U.S. patentapplication Ser. No. 10/465,520 filed Jun. 19, 2003 entitled “MethodsAnd Systems Involving Subcutaneous Electrode Positioning Relative To AHeart”, which are hereby incorporated herein by reference.

[0033] Certain configurations illustrated herein are generally describedas capable of implementing various functions traditionally performed byan implantable cardioverter/defibrillator (ICD), and may operate innumerous cardioversion/defibrillation modes as are known in the art.Examples of ICD circuitry, structures and functionality, aspects ofwhich may be incorporated in an ITCS device in accordance with thepresent invention, are disclosed in commonly owned U.S. Pat. Nos.5,133,353; 5,179,945; 5,314,459; 5,318,597; 5,620,466; and 5,662,688,which are hereby incorporated herein by reference in their respectiveentireties.

[0034] In particular configurations, systems and methods may performfunctions traditionally performed by pacemakers, such as providingvarious pacing therapies as are known in the art, in addition tocardioversion/defibrillation therapies. Examples of pacemaker circuitry,structures and functionality, aspects of which may be incorporated in anITCS device in accordance with the present invention, are disclosed incommonly owned U.S. Pat. Nos. 4,562,841; 5,284,136; 5,376,106;5,036,849; 5,540,727; 5,836,987; 6,044,298; and 6,055,454, which arehereby incorporated herein by reference in their respective entireties.It is understood that ITCS device configurations may provide fornon-physiologic pacing support in addition to, or to the exclusion of,bradycardia and/or anti-tachycardia pacing therapies.

[0035] An ITCS device in accordance with the present invention mayimplement diagnostic and/or monitoring functions as well as providecardiac stimulation therapy. Examples of cardiac monitoring circuitry,structures and functionality, aspects of which may be incorporated in anITCS device in accordance with the present invention, are disclosed incommonly owned U.S. Pat. Nos. 5,313,953; 5,388,578; and 5,411,031, whichare hereby incorporated herein by reference in their respectiveentireties.

[0036] An ITCS device may be used to implement various diagnosticfunctions, which may involve performing rate-based, pattern andrate-based, and/or morphological tachyarrhythmia discriminationanalyses. Subcutaneous, cutaneous, and/or external sensors may beemployed to acquire physiologic and non-physiologic information forpurposes of enhancing tachyarrhythmia detection and termination. It isunderstood that configurations, features, and combination of featuresdescribed in the present disclosure may be implemented in a wide rangeof implantable medical devices, and that such embodiments and featuresare not limited to the particular devices described herein.

[0037] Referring now to FIGS. 1A and 1B of the drawings, there is showna configuration of a transthoracic cardiac sensing and/or stimulation(ITCS) device having components implanted in the chest region of apatient at different locations. In the particular configuration shown inFIGS. 1A and 1B, the ITCS device includes a housing 102 within whichvarious cardiac sensing, detection, processing, and energy deliverycircuitry may be housed. It is understood that the components andfunctionality depicted in the figures and described herein may beimplemented in hardware, software, or a combination of hardware andsoftware. It is further understood that the components and functionalitydepicted as separate or discrete blocks/elements in the figures may beimplemented in combination with other components and functionality, andthat the depiction of such components and functionality in individual orintegral form is for purposes of clarity of explanation, and not oflimitation.

[0038] Communications circuitry is disposed within the housing 102 forfacilitating communication between the ITCS device and an externalcommunication device, such as a portable or bed-side communicationstation, patient-carried/worn communication station, or externalprogrammer, for example. The communications circuitry may alsofacilitate unidirectional or bidirectional communication with one ormore external, cutaneous, or subcutaneous physiologic or non-physiologicsensors. The housing 102 is typically configured to include one or moreelectrodes (e.g., can electrode and/or indifferent electrode). Althoughthe housing 102 is typically configured as an active can, it isappreciated that a non-active can configuration may be implemented, inwhich case at least two electrodes spaced apart from the housing 102 areemployed.

[0039] In the configuration shown in FIGS. 1A and 1B, a subcutaneouselectrode 104 may be positioned under the skin in the chest region andsituated distal from the housing 102. The subcutaneous and, ifapplicable, housing electrode(s) may be positioned about the heart atvarious locations and orientations, such as at various anterior and/orposterior locations relative to the heart. The subcutaneous electrode104 is coupled to circuitry within the housing 102 via a lead assembly106. One or more conductors (e.g., coils or cables) are provided withinthe lead assembly 106 and electrically couple the subcutaneous electrode104 with circuitry in the housing 102. One or more sense, sense/pace ordefibrillation electrodes may be situated on the elongated structure ofthe electrode support, the housing 102, and/or the distal electrodeassembly (shown as subcutaneous electrode 104 in the configuration shownin FIGS. 1A and 1B).

[0040] In one configuration, the lead assembly 106 is generally flexibleand has a construction similar to conventional implantable, medicalelectrical leads (e.g., defibrillation leads or combineddefibrillation/pacing leads). In another configuration, the leadassembly 106 is constructed to be somewhat flexible, yet has an elastic,spring, or mechanical memory that retains a desired configuration afterbeing shaped or manipulated by a clinician. For example, the leadassembly 106 may incorporate a gooseneck or braid system that may bedistorted under manual force to take on a desired shape. In this manner,the lead assembly 106 may be shape-fit to accommodate the uniqueanatomical configuration of a given patient, and generally retains acustomized shape after implantation. Shaping of the lead assembly 106according to this configuration may occur prior to, and during, ITCSdevice implantation.

[0041] In accordance with a further configuration, the lead assembly 106includes a rigid electrode support assembly, such as a rigid elongatedstructure that positionally stabilizes the subcutaneous electrode 104with respect to the housing 102. In this configuration, the rigidity ofthe elongated structure maintains a desired spacing between thesubcutaneous electrode 104 and the housing 102, and a desiredorientation of the subcutaneous electrode 104/housing 102 relative tothe patient's heart. The elongated structure may be formed from astructural plastic, composite or metallic material, and includes, or iscovered by, a biocompatible material. Appropriate electrical isolationbetween the housing 102 and subcutaneous electrode 104 is provided incases where the elongated structure is formed from an electricallyconductive material, such as metal.

[0042] In one configuration, the rigid electrode support assembly andthe housing 102 define a unitary structure (e.g., a singlehousing/unit). The electronic components and electrodeconductors/connectors are disposed within or on the unitary ITCS devicehousing/electrode support assembly. At least two electrodes aresupported on the unitary structure near opposing ends of thehousing/electrode support assembly. The unitary structure may have anarcuate or angled shape, for example.

[0043] According to another configuration, the rigid electrode supportassembly defines a physically separable unit relative to the housing102. The rigid electrode support assembly includes mechanical andelectrical couplings that facilitate mating engagement withcorresponding mechanical and electrical couplings of the housing 102.For example, a header block arrangement may be configured to includeboth electrical and mechanical couplings that provide for mechanical andelectrical connections between the rigid electrode support assembly andhousing 102. The header block arrangement may be provided on the housing102 or the rigid electrode support assembly. Alternatively, amechanical/electrical coupler may be used to establish mechanical andelectrical connections between the rigid electrode support assembly andhousing 102. In such a configuration, a variety of different electrodesupport assemblies of varying shapes, sizes, and electrodeconfigurations may be made available for physically and electricallyconnecting to a standard ITCS device housing 102.

[0044] It is noted that the electrodes and the lead assembly 106 may beconfigured to assume a variety of shapes. For example, the lead assembly106 may have a wedge, chevron, flattened oval, or a ribbon shape, andthe subcutaneous electrode 104 may include a number of spacedelectrodes, such as an array or band of electrodes. Moreover, two ormore subcutaneous electrodes 104 may be mounted to multiple electrodesupport assemblies 106 to achieve a desired spaced relationship amongstsubcutaneous electrodes 104.

[0045] An ITCS device may incorporate circuitry, structures andfunctionality of the subcutaneous implantable medical devices disclosedin commonly owned U.S. Pat. Nos. 5,203,348; 5,230,337; 5,360,442;5,366,496; 5,397,342; 5,391,200; 5,545,202; 5,603,732; and 5,916,243,which are hereby incorporated herein by reference in their respectiveentireties.

[0046]FIG. 1C is a block diagram depicting various components of an ITCSdevice in accordance with one configuration. According to thisconfiguration, the ITCS device incorporates a processor-based controlsystem 205 which includes a micro-processor 206 coupled to appropriatememory (volatile and non-volatile) 209, it being understood that anylogic-based control architecture may be used. The control system 205 iscoupled to circuitry and components to sense, detect, and analyzeelectrical signals produced by the heart and deliver electricalstimulation energy to the heart under predetermined conditions to treatcardiac arrhythmias. In certain configurations, the control system 205and associated components also provide pacing therapy to the heart. Theelectrical energy delivered by the ITCS device may be in the form of lowenergy pacing pulses or high-energy pulses for cardioversion ordefibrillation.

[0047] Cardiac signals are sensed using the subcutaneous electrode(s)214 and the can or indifferent electrode 207 provided on the ITCS devicehousing. Cardiac signals may also be sensed using only the subcutaneouselectrodes 214, such as in a non-active can configuration. As such,unipolar, bipolar, or combined unipolar/bipolar electrode configurationsas well as multi-element electrodes and combinations of noise cancelingand standard electrodes may be employed. The sensed cardiac signals arereceived by sensing circuitry 204, which includes sense amplificationcircuitry and may also include filtering circuitry and ananalog-to-digital (A/D) converter. The sensed cardiac signals processedby the sensing circuitry 204 may be received by noise reductioncircuitry 203, which may further reduce noise before signals are sent tothe detection circuitry 202.

[0048] Noise reduction circuitry 203 may also be incorporated aftersensing circuitry 202 in cases where high power or computationallyintensive noise reduction algorithms are required. The noise reductioncircuitry 203, by way of amplifiers used to perform operations with theelectrode signals, may also perform the function of the sensingcircuitry 204. Combining the functions of sensing circuitry 204 andnoise reduction circuitry 203 may be useful to minimize the necessarycomponentry and lower the power requirements of the system.

[0049] In the illustrative configuration shown in FIG. 1C, the detectioncircuitry 202 is coupled to, or otherwise incorporates, noise reductioncircuitry 203. The noise reduction circuitry 203 operates to improve thesignal-to-noise ratio (SNR) of sensed cardiac signals by removing noisecontent of the sensed cardiac signals introduced from various sources.Typical types of transthoracic cardiac signal noise includes electricalnoise and noise produced from skeletal muscles, for example.

[0050] Detection circuitry 202 typically includes a signal processorthat coordinates analysis of the sensed cardiac signals and/or othersensor inputs to detect cardiac arrhythmias, such as, in particular,tachyarrhythmia. Rate based and/or morphological discriminationalgorithms may be implemented by the signal processor of the detectioncircuitry 202 to detect and verify the presence and severity of anarrhythmic episode. Examples of arrhythmia detection and discriminationcircuitry, structures, and techniques, aspects of which may beimplemented by an ITCS device in accordance with the present invention,are disclosed in commonly owned U.S. Pat. Nos. 5,301,677 and 6,438,410,which are hereby incorporated herein by reference in their respectiveentireties.

[0051] The detection circuitry 202 communicates cardiac signalinformation to the control system 205. Memory circuitry 209 of thecontrol system 205 contains parameters for operating in various sensing,defibrillation, and, if applicable, pacing modes, and stores dataindicative of cardiac signals received by the detection circuitry 202.The memory circuitry 209 may also be configured to store historical ECGand therapy data, which may be used for various purposes and transmittedto an external receiving device as needed or desired.

[0052] In certain configurations, the ITCS device may includediagnostics circuitry 210. The diagnostics circuitry 210 typicallyreceives input signals from the detection circuitry 202 and the sensingcircuitry 204. The diagnostics circuitry 210 provides diagnostics datato the control system 205, it being understood that the control system205 may incorporate all or part of the diagnostics circuitry 210 or itsfunctionality. The control system 205 may store and use informationprovided by the diagnostics circuitry 210 for a variety of diagnosticspurposes. This diagnostic information may be stored, for example,subsequent to a triggering event or at predetermined intervals, and mayinclude system diagnostics, such as power source status, therapydelivery history, and/or patient diagnostics. The diagnostic informationmay take the form of electrical signals or other sensor data acquiredimmediately prior to therapy delivery.

[0053] According to a configuration that provides cardioversion anddefibrillation therapies, the control system 205 processes cardiacsignal data received from the detection circuitry 202 and initiatesappropriate tachyarrhythmia therapies to terminate cardiac arrhythmicepisodes and return the heart to normal sinus rhythm. The control system205 is coupled to shock therapy circuitry 216. The shock therapycircuitry 216 is coupled to the subcutaneous electrode(s) 214 and thecan or indifferent electrode 207 of the ITCS device housing. Uponcommand, the shock therapy circuitry 216 delivers cardioversion anddefibrillation stimulation energy to the heart in accordance with aselected cardioversion or defibrillation therapy. In a lesssophisticated configuration, the shock therapy circuitry 216 iscontrolled to deliver defibrillation therapies, in contrast to aconfiguration that provides for delivery of both cardioversion anddefibrillation therapies. Examples of ICD high energy deliverycircuitry, structures and functionality, aspects of which may beincorporated in an ITCS device of a type that may benefit from aspectsof the present invention are disclosed in commonly owned U.S. Pat. Nos.5,372,606; 5,411,525; 5,468,254; and 5,634,938, which are herebyincorporated herein by reference in their respective entireties.

[0054] In accordance with another configuration, an ITCS device mayincorporate a cardiac pacing capability in addition to cardioversionand/or defibrillation capabilities. As is shown in dotted lines in FIG.1C, the ITCS device may include pacing therapy circuitry 230, which iscoupled to the control system 205 and the subcutaneous andcan/indifferent electrodes 214, 207. Upon command, the pacing therapycircuitry delivers pacing pulses to the heart in accordance with aselected pacing therapy. Control signals, developed in accordance with apacing regimen by pacemaker circuitry within the control system 205, areinitiated and transmitted to the pacing therapy circuitry 230 wherepacing pulses are generated. A pacing regimen may be modified by thecontrol system 205.

[0055] A number of cardiac pacing therapies may be useful in atransthoracic cardiac monitoring and/or stimulation device. Such cardiacpacing therapies may be delivered via the pacing therapy circuitry 230as shown in FIG. 1C. Alternatively, cardiac pacing therapies may bedelivered via the shock therapy circuitry 216, which effectivelyobviates the need for separate pacemaker circuitry.

[0056] The ITCS device shown in FIG. 1C is configured to receive signalsfrom one or more physiologic and/or non-physiologic sensors inaccordance with embodiments of the present invention. Depending on thetype of sensor employed, signals generated by the sensors may becommunicated to transducer circuitry coupled directly to the detectioncircuitry 202 or indirectly via the sensing circuitry 204. It is notedthat certain sensors may transmit sense data to the control system 205without processing by the detection circuitry 202.

[0057] Non-electrophysiological cardiac sensors may be coupled directlyto the detection circuitry 202 or indirectly via the sensing circuitry204. Non-electrophysiological cardiac sensors sense cardiac activitythat is non-electrophysiological in nature. Examples ofnon-electrophysiological cardiac sensors include blood oxygen sensors,transthoracic impedance sensors, blood volume sensors, acoustic sensorsand/or pressure transducers, and accelerometers. Signals from thesesensors are developed based on cardiac activity, but are not deriveddirectly from electrophysiological sources (e.g., R-waves or P-waves). Anon-electrophysiological cardiac sensor 261, as is illustrated in FIG.1C, may be connected to one or more of the sensing circuitry 204,detection circuitry 202 (connection not shown for clarity), and thecontrol system 205.

[0058] Communications circuitry 218 is coupled to the microprocessor 206of the control system 205. The communications circuitry 218 allows theITCS device to communicate with one or more receiving devices or systemssituated external to the ITCS device. By way of example, the ITCS devicemay communicate with a patient-worn, portable or bedside communicationsystem via the communications circuitry 218. In one configuration, oneor more physiologic or non-physiologic sensors (subcutaneous, cutaneous,or external of patient) may be equipped with a short-range wirelesscommunication interface, such as an interface conforming to a knowncommunications standard, such as Bluetooth or IEEE 802 standards. Dataacquired by such sensors may be communicated to the ITCS device via thecommunications circuitry 218. It is noted that physiologic ornon-physiologic sensors equipped with wireless transmitters ortransceivers may communicate with a receiving system external of thepatient.

[0059] The communications circuitry 218 may allow the ITCS device tocommunicate with an external programmer. In one configuration, thecommunications circuitry 218 and the programmer unit (not shown) use awire loop antenna and a radio frequency telemetric link, as is known inthe art, to receive and transmit signals and data between the programmerunit and communications circuitry 218. In this manner, programmingcommands and data are transferred between the ITCS device and theprogrammer unit during and after implant. Using a programmer, aphysician is able to set or modify various parameters used by the ITCSdevice. For example, a physician may set or modify parameters affectingsensing, detection, pacing, and defibrillation functions of the ITCSdevice, including pacing and cardioversion/defibrillation therapy modes.

[0060] Typically, the ITCS device is encased and hermetically sealed ina housing suitable for implanting in a human body as is known in theart. Power to the ITCS device is supplied by an electrochemical powersource 220 housed within the ITCS device. In one configuration, thepower source 220 includes a rechargeable battery. According to thisconfiguration, charging circuitry is coupled to the power source 220 tofacilitate repeated non-invasive charging of the power source 220. Thecommunications circuitry 218, or separate receiver circuitry, isconfigured to receive RF energy transmitted by an external RF energytransmitter. The ITCS device may, in addition to a rechargeable powersource, include a non-rechargeable battery. It is understood that arechargeable power source need not be used, in which case a long-lifenon-rechargeable battery is employed.

[0061]FIG. 1D illustrates a configuration of detection circuitry 302 ofan ITCS device, which includes one or both of rate detection circuitry310 and morphological analysis circuitry 312. Detection and verificationof arrhythmias may be accomplished using rate-based discriminationalgorithms as known in the art implemented by the rate detectioncircuitry 310. Arrhythmic episodes may also be detected and verified bymorphology-based analysis of sensed cardiac signals as is known in theart. Tiered or parallel arrhythmia discrimination algorithms may also beimplemented using both rate-based and morphologic-based approaches.Further, a rate and pattern-based arrhythmia detection anddiscrimination approach may be employed to detect and/or verifyarrhythmic episodes, such as by use of the approaches disclosed in U.S.Pat. Nos. 6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186,which are hereby incorporated herein by reference.

[0062] The detection circuitry 302, which is coupled to a microprocessor306, may be configured to incorporate, or communicate with, specializedcircuitry for processing sensed cardiac signals in manners particularlyuseful in a transthoracic cardiac sensing and/or stimulation device. Asis shown by way of example in FIG. 1D, the detection circuitry 302 mayreceive information from multiple physiologic and non-physiologicsensors. Transthoracic acoustics, for example, may be monitored using anappropriate acoustic sensor. Heart sounds, for example, may be detectedand processed by non-electrophysiologic cardiac sensor processingcircuitry 318 for a variety of purposes. The acoustics data istransmitted to the detection circuitry 302, via a hardwire or wirelesslink, and used to enhance cardiac signal detection and/or arrhythmiadetection. For example, acoustic information may be used in accordancewith the present invention to corroborate ECG rate-based discriminationof arrhythmias.

[0063] The detection circuitry 302 may also receive information from oneor more sensors that monitor skeletal muscle activity. In addition tocardiac activity signals, transthoracic electrodes readily detectskeletal muscle signals. Such skeletal muscle signals may be used todetermine the activity level of the patient. In the context of cardiacsignal detection, such skeletal muscle signals are considered artifactsof the cardiac activity signal, which may be viewed as noise. Processingcircuitry 316 receives signals from one or more skeletal muscle sensors,and transmits processed skeletal muscle signal data to the detectioncircuitry 302. This data may be used to discriminate normal cardiacsinus rhythm with skeletal muscle noise from cardiac arrhythmias.

[0064] As was previously discussed, the detection circuitry 302 iscoupled to, or otherwise incorporates, noise-processing circuitry 314.The noise processing circuitry 314 processes sensed cardiac signals toimprove the SNR of sensed cardiac signals by reducing noise content ofthe sensed cardiac signals.

[0065] The components, functionality, and structural configurationsdepicted in FIGS. 1A-1D are intended to provide an understanding ofvarious features and combination of features that may be incorporated inan ITCS device. It is understood that a wide variety of ITCS and otherimplantable cardiac monitoring and/or stimulation device configurationsare contemplated, ranging from relatively sophisticated to relativelysimple designs. As such, particular ITCS or cardiac monitoring and/orstimulation device configurations may include particular features asdescribed herein, while other such device configurations may excludeparticular features described herein.

[0066] In accordance with embodiments of the invention, an ITCS devicemay be implemented to include a subcutaneous electrode system thatprovides for one or both of cardiac sensing and arrhythmia therapydelivery. According to one approach, an ITCS device may be implementedas a chronically implantable system that performs monitoring, diagnosticand/or therapeutic functions. The ITCS device may automatically detectand treat cardiac arrhythmias.

[0067] In one configuration, an ITCS device includes a pulse generatorand one or more electrodes that are implanted subcutaneously in thechest region of the body, such as in the anterior thoracic region of thebody. The ITCS device may be used to provide atrial and/or ventriculartherapy for bradycardia and tachycardia arrhythmias. Tachyarrhythmiatherapy may include cardioversion, defibrillation and anti-tachycardiapacing (ATP), for example, to treat atrial or ventricular tachycardia orfibrillation. Bradycardia therapy may include temporary post-shockpacing for bradycardia or asystole. Methods and systems for implementingpost-shock pacing for bradycardia or asystole are described in commonlyowned U.S. Patent Application entitled “Subcutaneous Cardiac StimulatorEmploying Post-Shock Transthoracic Asystole Prevention Pacing, Ser. No.10/377,274, filed on Feb. 28, 2003, which is incorporated herein byreference in its entirety.

[0068] In one configuration, an ITCS device according to one approachmay utilize conventional pulse generator and subcutaneous electrodeimplant techniques. The pulse generator device and electrodes may bechronically implanted subcutaneously. Such an ITCS may be used toautomatically detect and treat arrhythmias similarly to conventionalimplantable systems. In another configuration, the ITCS device mayinclude a unitary structure (e.g., a single housing/unit). Theelectronic components and electrode conductors/connectors are disposedwithin or on the unitary ITCS device housing/electrode support assembly.

[0069] The ITCS device contains the electronics and may be similar to aconventional implantable defibrillator. High voltage shock therapy maybe delivered between two or more electrodes, one of which may be thepulse generator housing (e.g., can), placed subcutaneously in thethoracic region of the body.

[0070] Additionally or alternatively, the ITCS device may also providelower energy electrical stimulation for bradycardia therapy. The ITCSdevice may provide brady pacing similarly to a conventional pacemaker.The ITCS device may provide temporary post-shock pacing for bradycardiaor asystole. Sensing and/or pacing may be accomplished using sense/paceelectrodes positioned on an electrode subsystem also incorporating shockelectrodes, or by separate electrodes implanted subcutaneously.

[0071] The ITCS device may detect a variety of physiological signalsthat may be used in connection with various diagnostic, therapeutic ormonitoring implementations in accordance with the present invention. Forexample, the ITCS device may include sensors or circuitry for detectingpulse pressure signals, blood oxygen level, heart sounds, cardiacacceleration, and other non-electrophysiological signals related tocardiac activity. In one embodiment, the ITCS device sensesintrathoracic impedance, from which various respiratory parameters maybe derived, including, for example, respiratory tidal volume and minuteventilation. Sensors and associated circuitry may be incorporated inconnection with an ITCS device for detecting one or more body movementor body position related signals. For example, accelerometers and GPSdevices may be employed to detect patient activity, patient location,body orientation, or torso position.

[0072] The ITCS device may be used within the structure of an advancedpatient management (APM) system. Advanced patient management systems mayallow physicians to remotely and automatically monitor cardiac andrespiratory functions, as well as other patient conditions. In oneexample, implantable cardiac rhythm management systems, such as cardiacpacemakers, defibrillators, and resynchronization devices, may beequipped with various telecommunications and information technologiesthat enable real-time data collection, diagnosis, and treatment of thepatient. Various embodiments described herein may be used in connectionwith advanced patient management. Methods, structures, and/or techniquesdescribed herein, which may be adapted to provide for remotepatient/device monitoring, diagnosis, therapy, or other APM relatedmethodologies, may incorporate features of one or more of the followingreferences: U.S. Pat. Nos. 6,221,011; 6,270,457; 6,277,072; 6,280,380;6,312,378; 6,336,903; 6,358,203; 6,368,284; 6,398,728; and 6,440,066,which are hereby incorporated herein by reference.

[0073] An ITCS device according to one approach provides an easy toimplant therapeutic, diagnostic or monitoring system. The ITCS systemmay be implanted without the need for intravenous or intrathoracicaccess, providing a simpler, less invasive implant procedure andminimizing lead and surgical complications. In addition, this systemwould have advantages for use in patients for whom transvenous leadsystems cause complications. Such complications include, but are notlimited to, surgical complications, infection, insufficient vesselpatency, complications associated with the presence of artificialvalves, and limitations in pediatric patients due to patient growth,among others. An ITCS system according to this approach is distinct fromconventional approaches in that it may be configured to include acombination of two or more electrode subsystems that are implantedsubcutaneously in the anterior thorax.

[0074] In one configuration, as is illustrated in FIG. 2, electrodesubsystems of an ITCS system are arranged about a patient's heart 510.The ITCS system includes a first electrode subsystem, comprising a canelectrode 502, and a second electrode subsystem 504 that includes atleast two electrodes or at least one multi-element electrode. The secondelectrode subsystem 504 may include a number of electrodes used forsensing and/or electrical stimulation, and may also includenon-electrophysiologic sensors.

[0075] In various configurations, the second electrode subsystem 504 mayinclude a combination of electrodes. The combination of electrodes ofthe second electrode subsystem 504 may include coil electrodes, tipelectrodes, ring electrodes, multi-element coils, spiral coils, spiralcoils mounted on non-conductive backing, screen patch electrodes, andother electrode configurations. A suitable non-conductive backingmaterial is silicone rubber, for example.

[0076] The can electrode 502 is positioned on the housing 501 thatencloses the ITCS device electronics. In one embodiment, the canelectrode 502 includes the entirety of the external surface of housing501. In other embodiments, various portions of the housing 501 may beelectrically isolated from the can electrode 502 or from tissue. Forexample, the active area of the can electrode 502 may include all or aportion of either the anterior or posterior surface of the housing 501to direct current flow in a manner advantageous for cardiac sensingand/or stimulation.

[0077] In accordance with one embodiment, the housing 501 may resemblethat of a conventional implantable ICD, is approximately 20-100 cc involume, with a thickness of 0.4 to 2 cm and with a surface area on eachface of approximately 30 to 100 cm². As previously discussed, portionsof the housing may be electrically isolated from tissue to optimallydirect current flow. For example, portions of the housing 501 may becovered with a non-conductive, or otherwise electrically resistive,material to direct current flow. Suitable non-conductive materialcoatings include those formed from silicone rubber, polyurethane, orparylene, for example.

[0078] In addition, or alternatively, all or portions of the housing 501may be treated to change the electrical conductivity characteristicsthereof for purposes of optimally directing current flow. Various knowntechniques may be employed to modify the surface conductivitycharacteristics of the housing 501, such as by increasing or decreasingsurface conductivity, to optimize current flow. Such techniques mayinclude those that mechanically or chemically alter the surface of thehousing 501 to achieve desired electrical conductivity characteristics.

[0079] As was discussed above, cardiac signals collected fromsubcutaneously implanted electrodes may be corrupted by noise. Inaddition, certain noise sources have frequency characteristics similarto those of the cardiac signal. Such noise may lead to over sensing andspurious shocks. Due to the possibility of relatively high amplitude ofthe noise signal and overlapping frequency content, filtering alone doesnot lead to complete suppression of the noise. In addition, filterperformance is not generally sufficiently robust against the entireclass of noises encountered. Further, known adaptive filteringapproaches require a reference signal that is often unknown forsituations when a patient experiences VF or high amplitude noise.

[0080] In accordance with one approach of the present invention, an ITCSdevice may be implemented to discriminate cardiac signals within a groupof separated signals, such as those obtained from a blind sourceseparation (BSS) technique. Devices and methods of blind sourceseparation are further described in commonly owned U.S. patentapplication Ser. No. 10/741,814, filed Dec. 19, 2003, herebyincorporated herein by reference. Devices and methods associated withanother useful signal separation approach that uses noise cancelingelectrodes are further described in commonly owned U.S. patentapplication Ser. No. 10/738,608, filed Dec. 17, 2003, herebyincorporated herein by reference.

[0081] Information from a non-electrophysiologic sensor 503, such asthose described previously, may be used to improve the accuracy ofarrhythmia discrimination, such as ECG or other rate-based arrhythmiadiscrimination. A signal independent of cardiac electrical activity,such as an acoustic signal of cardiac heart-sounds, an accelerometer, ablood sensor, or other non-electrophysiologic source sensor, may be usedto improve the detection and discrimination of arrhythmia from normalsinus rhythm (NSR) in the presence of noise. The non-electrophysiologicsensor 503 may be provided in or on the housing 501, as illustrated inFIG. 2, or may be provided as part of the second electrode subsystem504, as described earlier. It is also contemplated that thenon-electrophysiologic sensor 503 may be directly coupled to the housing501 using an additional lead, or be wirelessly coupled as described withreference to FIGS. 1C and 1D.

[0082] In an embodiment of the present invention, heart sounds are usedto aid in signal discrimination when detecting various heart rhythms inthe presence of electrical noise and/or electrocardiographic artifacts.Because the additional discriminating non-electrophysiologic signal istime correlated with respect to the cardiac electrophysiologicalsignals, the non-electrophysiologic signal may provide information abouta patient's rhythm state even in the presence of electrical noise and/orelectrocardiographic artifacts. For example, the non-electrophysiologicsignal may be used to verify that the ECG signal contains a cardiacsignal having a QRS complex, and only ECG signals with QRS complexes areverified ECG signals. Subsequent analysis may require that only verifiedECG signals are used for calculations of, for example, heart rate. Thisprovides for more robust algorithms that are less susceptible tocontamination from electrical interference and noise.

[0083] In one embodiment, a subcutaneous sensor, such as anaccelerometer or acoustic transducer, may be used to detect heartsounds. The heart sounds may be used together with rate, curvature, andother ECG information to discriminate normal sinus with electrical noisefrom potentially lethal arrhythmias such as ventricular tachycardia andventricular fibrillation. An ITCS device may utilize one or more of thepresence, characteristics, and frequency of occurrence of the heartsound combined with ECG information when performing signal or rhythmdiscrimination.

[0084] A heart rate determined from the ECG signal may, for example, beanalyzed along with heart sound information for diagnostic purposes.High ECG heart rate detection along with normal rate heart sounds wouldindicate the presence of noise in the ECG signal. High ECG heart ratedetection along with modified heart sounds would indicate a potentiallylethal arrhythmia. It is noted that ECG morphology or other techniquescould replace rate in the example above. It should also be noted thatother sensor derived signals could replace heart sounds. For example,impedance, pulse pressure, blood volume/flow, or cardiac accelerationscould be used.

[0085] Various types of acoustic sensors may be used to detect heartsounds. Examples of such acoustic sensors include diaphragm basedacoustic sensors, MEMS-based acoustic sensors such as a MEMS-basedacoustic transducer, fiber optic acoustic sensors, piezoelectricsensors, and accelerometer based acoustic sensors and arrays. Thesesensors may be used to detect the audio frequency pressure wavesassociated with the heart sounds, and may also be used to detect othernon-electrophysiologic cardiac related signals.

[0086] The presence of cardiac pulse, or heartbeat, in a patient isgenerally detected by palpating the patient's neck and sensing changesin the volume of the patient's carotid artery due to blood pumped fromthe patient's heart. A graph of a carotid pulse signal 810,representative of the physical expansion and contraction of a patient'scarotid artery during two consecutive pulses, or heartbeats, is shown atthe top of FIG. 3. When the heart's ventricles contract during aheartbeat, a pressure wave is sent throughout the patient's peripheralcirculation system. The carotid pulse signal 810 shown in FIG. 3 riseswith the ventricular ejection of blood at systole and peaks when thepressure wave from the heart reaches a maximum. The carotid pulse signal810 falls off again as the pressure subsides toward the end of eachpulse.

[0087] The opening and closing of the patient's heart valves during aheartbeat causes high-frequency vibrations in the adjacent heart walland blood vessels. These vibrations can be heard in the patient's bodyas heart sounds, and may be detected by sensors, as described earlier. Aconventional phonocardiogram (PCG) transducer placed on a patientconverts the acoustical energy of the heart sounds to electrical energy,resulting in a PCG waveform 820 that may be recorded and displayed, asshown by the graph in the upper middle portion of FIG. 3.

[0088] As indicated by the PCG waveform 820 shown in FIG. 3, a typicalheartbeat produces two main heart sounds. A first heart sound 830,denoted S1, is generated by vibration generally associated with theclosure of the tricuspid and mitral valves at the beginning of systole.Typically, the heart sound 830 is about 14 milliseconds long andcontains frequencies up to approximately 500 Hz. A second heart sound840, denoted S2, is generally associated with vibrations resulting fromthe closure of the aortic and pulmonary valves at the end of systole.While the duration of the second heart sound 840 is typically shorterthan the first heart sound 830, the spectral bandwidth of the secondheart sound 840 is typically larger than that of the first heart sound830.

[0089] An electrocardiogram (ECG) waveform 850 describes the electricalactivity of a patient's heart. The graph in the lower middle portion ofFIG. 3 illustrates an example of the ECG waveform 850 for two heartbeatsand corresponds in time with the carotid pulse signal 810 and PCGwaveform 820 also shown in FIG. 3. Referring to the first shownheartbeat, the portion of the ECG waveform 850 representingdepolarization of the atrial muscle fibers is referred to as the “P”wave. Depolarization of the ventricular muscle fibers is collectivelyrepresented by the “Q.” “R,” and “S” waves of the ECG waveform, referredto as the QRS complex. Finally, the portion of the waveform representingrepolarization of the ventricular muscle fibers is known as the “T”wave. Between heartbeats, the ECG waveform 850 returns to anisopotential level.

[0090] Fluctuations in a patient's transthoracic impedance signal 860also correlate with blood flow that occurs with each cardiac pulse wave.The bottom graph of FIG. 3 illustrates an example of a filteredtransthoracic impedance signal 860 for a patient in which fluctuationsin impedance correspond in time with the carotid pulse signal 810, thePCG waveform 820, and ECG waveform 850, also shown in FIG. 3.

[0091] Referring now to FIG. 4, in another embodiment of the presentinvention involving heart sounds, such sounds may be used fordiscrimination of arrhythmia from normal sinus rhythm. FIG. 4 is a graphdepicting two consecutive PQRS complexes in the ECG signal 850 and theirassociated non-electrophysiological components developed from anaccelerometer signal 835. Also illustrated is a detection window 870that is used to evaluate correlation of the signals in accordance withan embodiment of the present invention. As is illustrated in FIG. 4, anS1 heart sound 832, and an S1 heart sound 834 are, in general, closelytime correlated with a QRS complex 852 and a QRS complex 854respectively. The S1 heart sound 832, an S2 heart sound 833, and the S1heart sound 834 are illustrated as detected from an internally implantedaccelerometer. The S1 heart sound may provide a close time correlationwith cardiac signals but not with noise and artifact signals. As such,heart sounds may be used to discriminate an arrhythmia from NSR.

[0092] In an embodiment of a method in accordance with the presentinvention, a method of arrhythmia detection uses the ECG signal todefine a detection window. A non-electrophysiological source signal isthen evaluated within the detection window for cardiac information. Ifthe non-electrophysiological source signal includes a cardiac eventwithin the window, then the ECG signal is corroborated as correspondingto a cardiac event. This may be used, for example, in a rate-basedarrhythmia detection algorithm to provide a more robust rate than therate calculated if only ECG information is used. The algorithm may, forexample, only count ECG identified heart beats if the heart beats arecorroborated by an associated non-electrophysiologically sensed heartbeat.

[0093] An ITCS device may be implemented to include signal processingcircuitry and/or signal processing software as illustrated in FIGS. 1Cand 1D. With continued reference to FIG. 4, signal processing may beused to correlate heart sounds, such as the S1 heart sound, with R-wavepeaks or other QRS complex features to provide discrimination ofarrhythmias from NSR in the presence of noise.

[0094] In the approach illustrated in FIG. 4, an examination ordetection window 870 is defined to start at a start time 875, based onthe Q point of the QRS complex 852. The ITCS algorithm then searches theaccelerometer signal 835 within the detection window 870 for the S1heart sound 832. The algorithm may also look for time correlationbetween peak amplitudes of the S1 heart sound 832 and the peak R of theQRS complex 852. For example, the ECG signal 850 has an R-wave peak 856falling within the examination window 870, and an R-wave peak 858falling within an examination window 872. The R-wave peak 856 fallingwithin the examination window 870 produces a large correlation value,indicating that the ECG signal 850 is time correlated to the S1 heartsound signal 832 within the examination window 870. Similarly, theR-wave peak 858 falling within the examination window 872 produces alarge correlation value, indicating that the ECG signal 850 is timecorrelated to the S1 heart sound signal 834 within the examinationwindow 872. Heart rate, for example, may be determined betweensuccessive heart beats with large correlation values of the QRScomplexes 852, 854 and their associated S1 heart sounds 832, 834.

[0095] Referring now to FIG. 5, methods of signal discrimination inaccordance with the present invention are illustrated in a flowchart900. Electrocardiogram signals 902 are received at a subcutaneousnon-intrathoracic location. The electrocardiogram signals 902 mayinclude a cardiac signal and one or both of noise andelectrocardiographic artifacts. Non-electrophysiologic signals 904associated with a non-electrophysiological cardiac source are alsoreceived to provide cardiac function information that isnon-electrophysiologic in nature, such as heart sound information, bloodflow information, blood oxygen information, and information from othernon-electrophysiologic sensors described earlier. Both theelectrocardiogram signals 902 and non-electrophysiologic signals 904 areused to discriminate between normal sinus rhythm and an arrhythmia viaseveral optional paths, as is illustrated in the flowchart 900 of FIG.5.

[0096] An arrhythmia may be detected using the electrocardiogram signals902, and the presence of the arrhythmia may be verified using acomparison 903 of the electrocardiogram signals 902 to thenon-electrophysiologic signals 904. Temporal relationships between theelectrocardiogram signals 902 and non-electrophysiologic signals 904 maybe determined such as by using a comparison 905 of morphologies 907,909of the electrocardiogram signals 902 and the non-electrophysiologicsignals 904 respectively.

[0097] A detection window 906 may be initiated in response to receivingthe electrocardiogram signal 902, and used to determine whether thenon-electrophysiologic signal 904 is received at a time falling withinthe detection window 906, such as by using a correlation 911. The starttime of the detection window 906 used for confirmation may be associatedwith an inflection point of the electrocardiogram signal, such as amaxima or a minima or any other suitable morphological attribute.

[0098] Heart rates may be computed based on both a succession ofelectrocardiogram signals 902 and a succession of non-electrophysiologicsignals 904. An ECG rate 908 and a signal rate 916 may be used todiscriminate between normal sinus rhythm and the arrhythmia. The ECGrate 908 may be compared with an arrhythmia threshold 910, and used todetermine presence/absence of an arrhythmia, such as in response to afirst rate exceeding a first arrhythmia threshold but having a signalrate 916, at a second rate, failing to exceed a second arrhythmiathreshold 918.

[0099] Using any path in the flow-chart 900, defibrillation therapydelivery may be inhibited 920 or treated 921 in response to detecting anarrhythmia using the electrocardiogram signals 902 and confirming orrejecting the arrhythmia using, for example, the comparison 903, acomparison 941, and/or the correlation 911.

[0100]FIG. 6 is a flow chart illustrating a method of multi-parameterarrhythmia discrimination in accordance with another embodiment of thepresent invention. An arrhythmia discrimination method 950 isillustrated that involves to sensing 951 an electrocardiogram signal ata subcutaneous non-intrathoracic location. A signal associated with anon-electrophysiological cardiac source is received 952 and subject toverification 953 to determine whether or not the sensedelectrocardiogram signal includes a cardiac signal. A cardiac arrhythmiais detected 954 using one of the sensed electrocardiogram signal and theverified cardiac signal. Treatment of the cardiac arrhythmia is withheld955 if the sensed signal is not the cardiac signal. A verificationmethodology according to this and other embodiments advantageouslyreduces or eliminates delivery of unnecessary cardiac shocks, byensuring that the sensed signal from which arrhythmia detection,confirmation, and therapy decisions are made is indeed a cardiac signal.

[0101] Approaches to cardiac signal discrimination described hereininvolve the use of a non-electrophysiologic signal to discriminateand/or verify an arrhythmia and its associated ECG signal. Becausesignals developed from non-electrophysiological cardiac sources, such asheart sounds, are not electrophysiological in nature, they are notsusceptible to the same noise sources as electrocardiogram signals.

[0102] An ITCS device may operate in a batch mode or adaptively,allowing for on-line or off-line implementation. To save power, thesystem may include the option for a hierarchical decision-making routinethat uses algorithms known in the art for identifying presence ofarrhythmias or noise in the collected signal and judiciously turning onand off the cardiac signal discrimination methods in accordance with thepresent invention.

[0103] Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

What is claimed is:
 1. An arrhythmia discrimination method, comprising:sensing electrocardiogram signals at a subcutaneous non-intrathoraciclocation; receiving signals associated with a non-electrophysiologicalcardiac source; verifying that the electrocardiogram signals comprise acardiac signal using the non-electrophysiological signal;discriminating, using the electrocardiogram signals andnon-electrophysiologic signals, between a normal sinus rhythm and acardiac arrhythmia; and withholding delivery of subcutaneousnon-intrathoracic cardiac stimulation therapy if the sensed signal isnot the cardiac signal.
 2. The method of claim 1, wherein discriminatingbetween normal sinus rhythm and the arrhythmia comprises: detecting thearrhythmia using the electrocardiogram signals; and verifying presenceof the arrhythmia using the non-electrophysiologic signals.
 3. Themethod of claim 1, wherein discriminating between normal sinus rhythmand the arrhythmia comprises: detecting the arrhythmia using theelectrocardiogram signals; determining temporal relationships betweenthe electrocardiogram signals and non-electrophysiologic signalsreceived while detecting the arrhythmia; and verifying presence of thearrhythmia based on the temporal relationships between theelectrocardiogram signals and non-electrophysiologic signals.
 4. Themethod of claim 1, wherein discriminating between normal sinus rhythmand the arrhythmia comprises: initiating a detection window in responseto receiving each electrocardiogram signal of a succession of theelectrocardiogram signals; and determining whether eachnon-electrophysiologic signal of a succession of thenon-electrophysiologic signals is received at a time falling within thedetection window.
 5. The method of claim 1, wherein discriminatingbetween normal sinus rhythm and the arrhythmia comprises: computing afirst rate based on successive electrocardiogram signals; computing asecond rate based on successive non-electrophysiologic signals; anddiscriminating between normal sinus rhythm and the arrhythmia using thefirst and second rates.
 6. The method of claim 1, wherein discriminatingbetween normal sinus rhythm and the arrhythmia comprises: computing afirst rate based on successive electrocardiogram signals; computing asecond rate based on successive non-electrophysiologic signals;comparing the first rate with a first arrhythmia threshold; comparingthe second rate with a second arrhythmia threshold; and determiningpresence of the arrhythmia in response to both the first and secondrates exceeding the first and second arrhythmia thresholds,respectively.
 7. The method of claim 1, wherein discriminating betweennormal sinus rhythm and the arrhythmia comprises: computing a first ratebased on successive electrocardiogram signals; computing a second ratebased on successive non-electrophysiologic signals; comparing the firstrate with a first arrhythmia threshold; comparing the second rate with asecond arrhythmia threshold; and determining absence of the arrhythmiain response to the first rate exceeding the first arrhythmia thresholdand the second rate failing to exceed the second arrhythmia threshold.8. The method of claim 1, wherein discriminating between normal sinusrhythm and the arrhythmia comprises: determining presence of thearrhythmia using a morphology of the electrocardiogram signals; andverifying presence of the arrhythmia using the non-electrophysiologicsignals.
 9. The method of claim 1, wherein the non-electrophysiologicsignals comprise heart sound signals.
 10. The method of claim 1, whereinthe non-electrophysiologic signals comprise subsonic acoustic signalsindicative of cardiac activity.
 11. The method of claim 1, wherein thenon-electrophysiologic signals comprise pulse pressure signals.
 12. Themethod of claim 1, wherein the non-electrophysiologic signals compriseimpedance signals indicative of cardiac activity.
 13. The method ofclaim 1, wherein the non-electrophysiologic signals comprise pulseoximetry signals.
 14. The method of claim 1, further comprisingdeclaring an arrhythmic episode in response to detecting the arrhythmiausing the electrocardiogram signals and detecting the arrhythmia usingthe non-electrophysiologic signals.
 15. The method of claim 1, furthercomprising enabling defibrillation therapy delivery in response todetecting the arrhythmia using the electrocardiogram signals anddetecting the arrhythmia using the non-electrophysiologic signals. 16.The method of claim 1, further comprising inhibiting defibrillationtherapy delivery in response to detecting the arrhythmia using theelectrocardiogram signals but not detecting the arrhythmia using thenon-electrophysiologic signals.
 17. An arrhythmia discrimination method,comprising: sensing an electrocardiogram signal at a subcutaneousnon-intrathoracic location; receiving a signal associated with anon-electrophysiological cardiac source; verifying that the sensedelectrocardiogram signal comprises a cardiac signal using thenon-electrophysiological signal; detecting a cardiac arrhythmia usingone of the sensed electrocardiogram signal and the verified cardiacsignal; and withholding treatment of the cardiac arrhythmia if thesensed signal is not the cardiac signal.
 18. The method of claim 17,further comprising: defining a detection window with a start timeassociated with an inflection point of the electrocardiogram signal; andevaluating the received non-electrophysiological signal within thedetection window.
 19. The method of claim 18, wherein the start time ofthe detection window is associated with a maxima or a minima of theelectrocardiogram signal.
 20. The method of claim 17, furthercomprising: computing a first heart-rate based on intervals betweensuccessive electrocardiogram signals; and computing a second heart-ratebased on intervals between successive non-electrophysiological cardiacsignals; wherein confirming presence of the cardiac arrhythmia comprisescomparing the first heart-rate to the second heart-rate.
 21. The methodof claim 17, wherein confirming presence of the cardiac arrhythmiacomprises performing a correlation between the electrocardiogram signaland the non-electrophysiological cardiac signal.
 22. The method of claim17, wherein the non-electrophysiological cardiac signal comprisesacoustic emission information.
 23. The method of claim 22, wherein theacoustic emission information comprises a temporal location of a peakheart-sound.
 24. The method of claim 17, wherein thenon-electrophysiological cardiac signal comprises cardiac accelerationinformation.
 25. The method of claim 17, wherein thenon-electrophysiological cardiac signal comprises pulse pressureinformation.
 26. The method of claim 17, wherein thenon-electrophysiological cardiac signal comprises blood-flowinformation.
 27. The method of claim 17, wherein thenon-electrophysiological cardiac signal comprises heart rateinformation.
 28. The method of claim 17, wherein thenon-electrophysiological cardiac signal comprises pulse oximetryinformation.
 29. The method of claim 17, wherein detecting presence ornon-presence of the cardiac arrhythmia comprises performing a rate basedanalysis of the electrocardiogram signal.
 30. The method of claim 17,wherein detecting presence or non-presence of the cardiac arrhythmiacomprises performing a morphology based analysis of theelectrocardiogram signal.
 31. The method of claim 17, further comprisingdelivering a cardiac therapy to treat the cardiac arrhythmia.
 32. Animplantable cardiac device, comprising: an implantable housing; anelectrode arrangement configured for subcutaneous non-intrathoracicplacement; detection circuitry provided in the housing and coupled tothe electrode arrangement, the detection circuitry configured to detectelectrocardiogram signals; a sensor configured to sense signalsassociated with a non-electrophysiological cardiac source; energydelivery circuitry coupled to the electrode arrangement; and a processorprovided in the housing and coupled to the detection circuitry, sensor,and energy delivery circuitry, the processor using thenon-electrophysiological signals to verify that the detectedelectrocardiogram signals comprise a cardiac signal, the processorwithholding treatment of the cardiac arrhythmia if the detectedelectrocardiogram signals do not comprise the cardiac signal.
 33. Thedevice of claim 32, wherein the energy delivery circuitry comprisesdefibrillation therapy circuitry.
 34. The device of claim 32, whereinthe energy delivery circuitry comprises pacing therapy circuitry. 35.The device of claim 32, wherein the sensor is provided in or on thehousing.
 36. The device of claim 32, wherein the sensor is provided inor on a lead coupled to the housing.
 37. The device of claim 32, whereinthe sensor comprises an accelerometer.
 38. The device of claim 32,wherein the sensor comprises a microphone.
 39. The device of claim 32,wherein the sensor comprises an acoustic transducer.
 40. The device ofclaim 32, wherein the sensor comprises a blood-flow transducer.
 41. Thedevice of claim 32, wherein the sensor comprises a pulse oximeter. 42.The device of claim 32, wherein the sensor comprisesphotoplethysmography circuitry.
 43. An implantable device, comprising:means for sensing an electrocardiogram signal at a subcutaneousnon-intrathoracic location; means for receiving a signal associated witha non-electrophysiological cardiac source; means for verifying that thesensed electrocardiogram signal comprises a cardiac signal using thenon-electrophysiological signal; means for detecting a cardiacarrhythmia using one of the sensed electrocardiogram signal and theverified cardiac signal; and means for withholding treatment of thecardiac arrhythmia if the sensed signal is not the cardiac signal. 44.The device of claim 43, wherein the discriminating means comprises:means for detecting the arrhythmia using the electrocardiogram signals;and means for verifying presence of the arrhythmia using thenon-electrophysiologic signals.
 45. The device of claim 43, comprisingmeans for implantably treating the cardiac arrhythmia.